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            <small>
              <a href="#Procedure">Procedure<br></a>
              <a href="#Abstract">Abstract<br></a>
              <a href="#Required_Reading">Required_Reading<br></a>
              <a href="#Keywords">Keywords<br></a>
              <a href="#Brief_I/O">Brief_I/O<br></a>
              <a href="#Detailed_Input">Detailed_Input<br></a>

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              <small>               <a href="#Detailed_Output">Detailed_Output<br></a>
              <a href="#Parameters">Parameters<br></a>
              <a href="#Exceptions">Exceptions<br></a>
              <a href="#Files">Files<br></a>
              <a href="#Particulars">Particulars<br></a>
              <a href="#Examples">Examples<br></a>

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              <small>               <a href="#Restrictions">Restrictions<br></a>
              <a href="#Literature_References">Literature_References<br></a>
              <a href="#Author_and_Institution">Author_and_Institution<br></a>
              <a href="#Version">Version<br></a>
              <a href="#Index_Entries">Index_Entries<br></a>
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<h4><a name="Procedure">Procedure</a></h4>
<PRE>
   void surfnm_c ( SpiceDouble        a, 
                   SpiceDouble        b, 
                   SpiceDouble        c, 
                   ConstSpiceDouble   point[3], 
                   SpiceDouble        normal[3] ) 
</PRE>
<h4><a name="Abstract">Abstract</a></h4>
<PRE>
 
   This routine computes the outward-pointing, unit normal vector 
   from a point on the surface of an ellipsoid. 
 </PRE>
<h4><a name="Required_Reading">Required_Reading</a></h4>
<PRE>
 
   None. 
 </PRE>
<h4><a name="Keywords">Keywords</a></h4>
<PRE>
 
   ELLIPSOID,  GEOMETRY 
 

</PRE>
<h4><a name="Brief_I/O">Brief_I/O</a></h4>
<PRE>
 
   VARIABLE  I/O  DESCRIPTION 
   --------  ---  -------------------------------------------------- 
   a          I   Length of the ellisoid semi-axis along the x-axis. 
   b          I   Length of the ellisoid semi-axis along the y-axis. 
   c          I   Length of the ellisoid semi-axis along the z-axis. 
   point      I   Body-fixed coordinates of a point on the ellipsoid 
   normal     O   Outward pointing unit normal to ellipsoid at point 
 </PRE>
<h4><a name="Detailed_Input">Detailed_Input</a></h4>
<PRE>
   a          This is the length of the semi-axis of the ellipsoid 
              that is parallel to the x-axis of the body-fixed 
              coordinate system. 

   b          This is the length of the semi-axis of the ellipsoid 
              that is parallel to the y-axis of the body-fixed 
              coordinate system. 

   c          This is the length of the semi-axis of the ellipsoid 
              that is parallel to the z-axis of the body-fixed 
              coordinate system. 

   point      This is a 3-vector giving the bodyfixed coordinates 
              of a point on the ellipsoid. In bodyfixed coordinates, 
              the semi-axes of the ellipsoid are aligned with the 
              x, y, and z-axes of the coordinate system. 
 </PRE>
<h4><a name="Detailed_Output">Detailed_Output</a></h4>
<PRE>
 
   normal    A unit vector pointing away from the ellipsoid and 
             normal to the ellipsoid at point. 
 </PRE>
<h4><a name="Parameters">Parameters</a></h4>
<PRE>
 
   None. 
 </PRE>
<h4><a name="Exceptions">Exceptions</a></h4>
<PRE>
 
   1) If any of the axes are non-positive, the error 
      SPICE(BADAXISLENGTH) will be signalled. 
 </PRE>
<h4><a name="Files">Files</a></h4>
<PRE>
 
   None. 
 </PRE>
<h4><a name="Particulars">Particulars</a></h4>
<PRE>
 
   This routine computes the outward pointing unit normal vector to 
   the ellipsoid having semi-axes of length a, b, and c from the 
   point point. 
 </PRE>
<h4><a name="Examples">Examples</a></h4>
<PRE>
 
   A typical use of <b>surfnm_c</b> would be to find the angle of incidence 
   of the light from the sun at a point on the surface of an 
   ellipsoid. 

   Let q be a 3-vector representing the rectangular body-fixed 
   coordinates of a point on the ellipsoid (we are assuming that 
   the axes of the ellipsoid are aligned with the axes of the 
   body fixed frame.)  Let v be the vector from q to the sun in 
   bodyfixed coordinates.  Then the following code fragment could 
   be used to compute angle of incidence of sunlight at q. 

      <b>surfnm_c</b>   ( a, b, c, q, nrml );

      incidn = <a href="vsep_c.html">vsep_c</a> ( v, nrml );
 
 </PRE>
<h4><a name="Restrictions">Restrictions</a></h4>
<PRE>
 
   It is assumed that the input point is indeed on the ellipsoid. 
   No checking for this is done. 
 </PRE>
<h4><a name="Literature_References">Literature_References</a></h4>
<PRE>
 
   None. 
 </PRE>
<h4><a name="Author_and_Institution">Author_and_Institution</a></h4>
<PRE>
 
   W.L. Taber      (JPL) 
   N.J. Bachman    (JPL)
   B.V. Semenov    (JPL)
   </PRE>
<h4><a name="Version">Version</a></h4>
<PRE>
 
   -CSPICE Version 1.3.1, 31-JAN-2008 (BVS)

      Removed '-Revisions' from the header.

   -CSPICE Version 1.3.0, 22-OCT-1998 (NJB)

      Made input vector const.

   -CSPICE Version 1.2.0, 08-FEB-1998 (NJB)
   
      Removed local variables used for temporary capture of outputs.

   -CSPICE Version 1.0.0, 25-OCT-1997 (NJB)
   
       Based on SPICELIB Version 1.2.0, 07-AUG-1996 (WLT)
</PRE>
<h4><a name="Index_Entries">Index_Entries</a></h4>
<PRE>
 
   surface normal vector on an ellipsoid 
 </PRE>
<h4>Link to routine surfnm_c source file <a href='../../../src/cspice/surfnm_c.c'>surfnm_c.c</a> </h4>

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   <pre>Wed Jun  9 13:05:31 2010</pre>

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